1. Field of the Invention
Illustrative embodiments described in this patent specification generally relate to a developing device used for a copier, a printer, a facsimile machine, or the like, an image forming apparatus including the developing device, and a process cartridge removably installable in the image forming apparatus.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
A developing device that uses two-component developer including toner and magnetic carrier is widely used in the image forming apparatuses. The developing device using the two-component developer often includes a developer bearing member (e.g., a developing roller) that rotates around a rotary shaft thereof and a casing that forms a developer container from which the developer is supplied to the developing roller. A developer conveyance path such as a supply path that supplies the developer to the developing roller while conveying the developer in a direction parallel to an axial direction of the developing roller and a conveyance screw rotated to convey the developer in the developer conveyance path in the direction parallel to the axial direction of the developing roller are provided within the casing of the developing device.
The developing device further includes a drive transmission unit composed of a gear train that transmits torque supplied externally to rotary bodies such as the developing roller and the conveyance screw provided to the developing device. The drive transmission unit is disposed outside a lateral wall of the casing of the developing device provided to one end of the casing in the axial direction. The drive transmission unit includes a drive input gear to which torque is supplied from a drive source included in the image forming apparatus. A rotary shaft of each of the rotary bodies of the developing device is extended outwardly from the interior of the casing of the developing device through the lateral wall of the casing in the axial direction, and the drive input gear that supplies the torque to the rotary bodies is fixed to the rotary shaft outside the lateral wall. The torque is transmitted, either directly or via other gears, from the drive input gear to drive gears respectively provided to the rotary bodies so that the rotary bodies are rotated.
In recent years, the size of the casing of the developing device has been reduced to provide a more compact developing device. However, if the drive input gear protrudes outwardly beyond the casing of the developing device in a direction perpendicular to the axial direction, the advantage of the reduction in the size of the developing device is lost.
It is conceivable that one of the drive gears of the rotary bodies is used as the drive input gear in order to prevent the drive input gear from protruding outwardly beyond the casing of the developing device. However, because the rotary bodies are rotated together with the rotary shafts thereof and the drive gears, and the rotary shafts are rotated relative to the casing of the developing device, there is some play in the rotary shafts, thereby causing displacement of the rotary shafts relative to the casing. In general, because the drive input gear transmits torque to all the rotary bodies provided to the developing device, it tends to generate a large amount of torque. Thus, displacement of the axial center of a rotary shaft of the drive input gear may cause unstable rotation of the drive input gear. For these reasons, the rotary shaft of the drive input gear needs to be fixed to the lateral wall of the casing of the developing device, and therefore one of the drive gears cannot be used as the drive input gear.
Positions of each of the rotary bodies such as the developing roller and the conveyance screw are determined such that the rotary bodies function properly in the developing device, and the casing of the developing device is formed to accommodate the rotary bodies. A stationary shaft member, that is, the rotary shaft of the drive input gear that supports the drive input gear, is provided at a position other than the positions of the rotary shafts of the rotary bodies, and the drive input gear having a diameter larger than a diameter of the stationary shaft member is fixed to the stationary shaft member. As a result, the drive input gear tends to protrude beyond the casing of the developing device in the direction perpendicular to the axial direction.
The above-described problem occurs not only in the case in which the stationary shaft member fixed to the lateral wall of the casing of the developing device is used as the rotary shaft of the drive input gear but also in a case in which a stationary shaft member fixed to the lateral wall of the casing of the developing device outside the casing in the axial direction to rotatably support the drive input gear is disposed at a position different from the rotary shafts of the rotary bodies.